In a computer system in which a plurality of servers share a storage device, as a technique for allocating IO (Input/Output) between the servers and the storage device, a technique explained below is known. The computer system includes the servers, an IO hub, an IO card connected to the IO hub, and a switch to which the servers and the IO hub are connected. The servers are set to be capable of occupying and using the IO card. The IO hub allocates a virtual MMIO (Memory Mapped Input/Output) address peculiar to a server to a physical MMIO address corresponding to the IO card, retains allocation information indicating a relation among the allocated virtual MMIO address, the physical MMIO address, and a server identifier, and, when an access request to the IO card is transmitted from the server, refers to the allocation information, extracts the server identifier from the access request, and identifies the server at an access request source. Further, the IO hub converts a DMA (Direct Memory Access) address included in a DMA request from the IO card into an address corresponding to an identified server at a DMA request source, and transfers the DMA request transmitted from the IO card to a memory space of the server on the basis of the converted address (PTL 1 and PTL 2).